From the sense of being above in rank and status. Essentially of the same meaning as 上 ‎(kami, “above”), but spelled 守 to convey a sense of “watching” or “overseeing”. This spelling is an example of jukujikun or ateji used for the meaning.

(archaic) in the first year of the new Meiji era government established in 1868, the title of the seven heads of all government bureaus except the 総裁局 ‎(Sōsai Kyoku, “Bureau of the President”), whose head was titled 総裁 ‎(Sōsai, “president” or “director general”)